1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to servo writing a disk drive using correction values that compensate for phase errors.
2. Description of the Prior Art
Disk drives for computer systems comprise a disk for storing data and a head actuated radially over the disk for writing data to and reading data from the disk. To effectuate the radial positioning of the head over the disk, the head is connected to the distal end of an actuator arm which is rotated about a pivot by a rotary actuator (e.g., a voice coil motor (VCM)). The disk is typically divided into a number of concentric, radially spaced tracks, where each track is divided into a number of data sectors. The disk is typically accessed a data sector at a time by positioning the head over the track which comprises the target data sector. As the disk spins, the head writes transitions (e.g., magnetic transitions) in the data sector to record data, and during read operations senses the transitions to recover the recorded data.
Accurate reproduction of the recorded data requires the head to be positioned very close to the centerline of the target data sector during both write and read operations. Thus, accessing a target data sector involves positioning or “seeking” the head to the target track, and then maintaining centerline “tracking” while data is written to or read from the disk. A closed loop servo system typically performs the seeking and tracking operations by controlling the rotary actuator in response to position information generated from the head.
A well known technique for generating the head position control information is to record servo information in servo sectors disbursed circumferentially about the disk, “embedded” with the data sectors. This is illustrated in FIG. 1 which shows a disk 2 comprising a number of concentric tracks 4 and a number of embedded servo sectors 60-6N. Each servo sector 61 comprises a preamble 8, a sync mark 10, servo data 12, and servo bursts 14. The preamble 8 comprises a periodic pattern which allows proper gain adjustment and timing synchronization of the read signal, and the sync mark 10 comprises a special pattern for symbol synchronizing to the servo data 12. The servo data 12 comprises identification information, such as sector identification data and a track address. The servo control system reads the track address during seeks to derive a coarse position for the head with respect to the target track. The track addresses are recorded using a phase coherent Gray code so that the track addresses can be accurately detected when the head is flying between tracks. The servo bursts 14 in the servo sectors 6 comprise groups of consecutive transitions (e.g., A, B, C and D bursts) which are recorded at precise intervals and offsets with respect to the track centerline. Fine head position control information is derived from the servo bursts 14 for use in centerline tracking while writing data to and reading data from the target track.
The embedded servo sectors 6 are written to the disk 2 as part of the manufacturing process. Conventionally, an external servo writer has been employed which writes the embedded servo sectors 6 to the disks by processing each head disk assembly (HDA) in an assembly line fashion. The external servo writers employ very precise head positioning mechanics, such as a laser interferometer, for positioning the head at precise radial locations with respect to previously servo-written tracks so as to achieve very high track densities. A clock track is written at the outer diameter of the disk, and a clock head inserted into the HDA to read the clock track in order to synchronize a phase locked loop (PLL) used to write the servo sectors at the appropriate circumferential location (so that the servo sectors are aligned radially across the disk as in FIG. 1). However, external servo writers are expensive and present a significant bottle neck to the manufacturing process. In addition, inserting a clock head into the HDA requires the servo writing process take place in a clean room environment to prevent particulate contamination.
Certain “self-servo writing” techniques have been disclosed wherein components internal to the disk drive are employed to perform the servo writing process which avoids the drawbacks of external servo writers including the need for a clean room since self servo writing occurs within the sealed HDA. One technique for self servo writing propagates sectors (servo sectors and optional timing sectors) from a reference track (e.g., written at the outer diameter) across the disk radius (e.g., toward the inner diameter). While servoing on the reference track, a PLL is synchronized to the sectors written on the reference track which is used to propagate the sectors to an adjacent track. The read element of the head is then placed over the adjacent track and the write element over the next track, the PLL synchronized to the sectors previously written on the adjacent track, and the sectors propagated to the next track. This process is repeated until the sectors are propagated across the entire disk surface. A problem with this propagation technique is that certain components of the phase error (e.g., spindle harmonics, suspension vibration, sensor noise, etc.) are amplified due to the peaking nature of the PLL transfer function. These components of the phase error will propagate from track-to-track and grow unboundedly resulting in “wandering” servo sectors across the disk surface (not radially aligned as in FIG. 1).
There is, therefore, a need to attenuate phase error propagation when self servo writing a disk drive by propagating sectors.